An apostolos or epistle is a writing directed or sent to a person or group of persons, usually a letter and a very formal, often didactic and elegant one. The letters from Apostles to Christians in the New Testament are often referred to as epistles.

Liturgical use

In the context of the Divine Liturgy or other liturgical service, epistle may refer more specifically to a particular passage from a New Testament epistle, or from the Acts of the Apostles, that is scheduled to be read on a certain day or at a certain occasion.

The liturgical book itself, often has the readings arranged in three parts: